Quantify the role of local radiation in Na I ionization evolution

Quantify how local radiation from host galaxies affects the ionization balance and neutral-sodium fraction of intervening Na I absorbers over the surveyed low-redshift interval.

Background

The paper argues that the declining metagalactic ionizing background could increase the neutral-sodium fraction toward the present epoch and thereby contribute to the observed increase in Na I incidence. However, local radiation from host galaxies may also influence the ionization state because Na I has a low ionization potential of 5.14 eV.

The authors state that the contribution of local radiation cannot be determined from the current analysis and requires detailed photoionization modelling.

References

Because the Na\ ionization potential is only 5.14~eV, local radiation from the host galaxy can also affect the ionization balance. Over the relatively small redshift interval considered here, such local radiation may mainly modify the normalization of the Na\ ionization conditions, but its role cannot be quantified without detailed photoionization modelling.

— The First Blind Survey of Intervening NaI Absorbers: Incidence and Cosmic Evolution with DESI  (2609.26794 - Anand et al., 22 Sep 2026) in Section 4.2, “Cosmic Evolution of Na at Low Redshifts”